.gamma.-Aminobutyric acid is a kind of amino acid that is produced by decarboxylation of glutamic acid in living organisms, and it is known that the acid plays an important role in the central nervous system as a neurotransmitter and, in addition, has a function to lower the blood pressure in animals including human beings.
Recently, it has been found that the content of .gamma.-aminobutyric acid in tea leaves may be increased to 5 to 10 times by anaerobically treating raw tea leaves. At present, such tea having an increased content of .gamma.-aminobutyric acid has been commercially sold by various makers as "Gabaron Tea" (trade name). "Gabaron Tea" has been proved to have an effect of curing hypertension by animal tests and has been utilized by persons suffering from hypertension as an easy means to cure hypertension without minding its side effect.
It is said necessary to perorally take a large amount of .gamma.-aminobutyric acid so as to lower the blood pressure in hypertension. For "Gabaron Tea", however, it is difficult to take a large amount of .gamma.-aminobutyric acid therefrom since the tea is diluted when extracted with hot water. This will be the reason why "Gabaron Tea" is considered ineffective against hypertension by some persons.
At present, .gamma.-aminobutyric acid is produced by fermentation, which, however, is not satisfactory in view of its cost. Under these situations, desired are the development of food materials from which a higher concentration of .gamma.-aminobutyric acid may be taken and the development of a method for producing a safe and low-priced .gamma.-aminobutyric acid.